Right, I forgot that the Descriptor object is statically initialized so it'll be consistent within the JVM.
I still think that in the context of serializable objects, (i.e. objects intended to be transported between JVMs in some way, shape or form) a consistent hashCode would be useful for a lot of cases (mine included). If it can't be consistent in the presence of unknown fields, perhaps a well-documented caution to that (relatively uncommon?) case would be more useful than completely punting on all cases. On May 18, 3:08 pm, Ron Reynolds <[email protected]> wrote: > the corner-stone of Hash* containers is: > (A.equals(B)) => (A.hashCode() == B.hashCode()) for all A, B. > > tho it's not explicitly stated it would seem to be implied that is within a > single JVM. > not sure if the code in question maintains that rule within a JVM (if not > that's > a big deal). > if so that would seem sufficient for all but the most distributed of Hash* > containers, such as where a client which is remote from the storage (i.e, in > another JVM) determines the bucket (based on hashCode()) to find that the > element in question has been placed into another bucket because the hashCode() > within the containing JVM has evaluated to another value. that's a pretty > far-fetched, but not unimaginable, situation. > > ________________________________ > From: Jason Hsueh <[email protected]> > To: Jay Booth <[email protected]> > Cc: Protocol Buffers <[email protected]> > Sent: Wed, May 18, 2011 12:00:19 PM > Subject: Re: [protobuf] Re: Generated hashcode() returns different values > across > JVM instances? > > Jumping in late to the thread, and I'm not really a Java person, so I may be > misunderstanding something here. But as far as I can tell, you are asking for > hashCode() to be a 'consistent' hash across processes. hashCode() as > implemented > is still useful within a single JVM, allowing you to use protobufs in HashMaps > based on content rather than object identity. That was the intended use case. > > On Wed, May 18, 2011 at 11:48 AM, Jay Booth <[email protected]> wrote: > > Well, that's your prerogative, I guess, but why even implement > > > > > > > > > > >hashcode at all then? Just inherit from object and you're getting > >effectively the same behavior. Is that what you're intending? > > >On May 16, 10:03 am, Pherl Liu <[email protected]> wrote: > >> We discussed internally and decided not to make the hashCode() > >> return deterministic result. If you need consistent hashcode in different > >> runs, use toByteString().hashCode(). > > >> Quoted from Kenton: > > >> Hashing the content of the descriptor would actually be incorrect, because > >> two descriptors with exactly the same content are still considered > >> different > >> types. Descriptors are compared by identity, hence they are hashed by > >> pointer. > > >> Removing the descriptor from the calculation would indeed make hashCode() > >> consistent between two runs of the same binary, and probably insignificant > >> runtime cost. Of course, once you do that, you will never be able to > >> introduce non-determinism again because people will depend on it. > > >> But there's a much bigger risk. People may actually start depending on > >> hashCode() returning consistent results between two different versions of > >> the binary, or two completely separate binaries that compile in the same > >> protocol, or -- most dangerously -- two different versions of the same > >> protocol (e.g. with fields added or removed). I think it would be very > >> difficult and limiting to make these guarantees, so I would be extremely > >> cautious about this. > > >> Certainly, there is no implementation of hashCode() that would be any safer > >> than .toByteString().hashCode(). So, I'd advise steering people to the > >> latter. Note that if unknown fields are present, the results may still be > >> inconsistent. However, there is no reasonable way to implement a > >> hashCode() > >> that is consistent in the presence of unknown fields. > > >> On Thu, May 12, 2011 at 5:32 AM, Ben Wright <[email protected]> wrote: > >> > I think we wrote those replies at the same time : ) > > >> > You're right, at the cost of some additional hash collisions, the > >> > simplest solution is to simply not include the type / descriptor in > >> > the hash calculation at all. > > >> > The best / least-collision solutions with good performance would be > >> > what I wrote in my previous post, but that requires that someone > >> > (presumably a current committer) with sufficient knowledge of the > >> > Descriptor types to have enough time to update the compiler and java > >> > libraries accordingly. > > >> > Any input from a committer for this issue? Seems the simple solution > >> > would take less than an hour to push into the stream and could make it > >> > into the next release. > > >> > On May 11, 5:25 pm, Ben Wright <[email protected]> wrote: > >> > > Alternatively... instead of putting the onus on the compiler, the > >> > > hashcode could be computed by the JVM at initialization time for the > >> > > Descriptor instance, (which would also help performance of dynamically > >> > > parsed Descriptor instance hashcode calls). > > >> > > i.e. > > >> > > private final int computedHashcode; > > >> > > public Descriptor() { > >> > > //initialization > > >> > > computedHashcode = do_compute_hashCode(); > > >> > > } > > >> > > public int hashCode() { > >> > > return computedHashcode; > > >> > > } > > >> > > punlic int do_compute_hashCode(){ > >> > > return // compute hashcode > > >> > > } > > >> > > This is all talking towards optimum performance implementation... the > >> > > real problem is the need for a hashCode implementation for Descriptor > >> > > based on the actual Descriptor's content... > > >> > > On May 11, 4:54 pm, Ben Wright <[email protected]> wrote: > > >> > > > Jay: > > >> > > > Using the class name to generate the hashcode is logically incorrect > >> > > > because the class name can be derived by the options java_package_ > >> > > > name and java_outer_classname. > > >> > > > Additionally (although less likely to matter), separate protocol > >> > > > buffer files can define an identical class names with different > >> > > > protocol buffers. > > >> > > > Lastly, and most importantly... > > >> > > > If the same Message is being used with generated code and with > >> > > > dynamic > >> > > > code, the hash code for the descriptor would still be identical if > >> > > > generated from the descriptor instance, whereas the dynamic usage > >> > > > does > >> > > > not have a classname from which to derive a hashcode. While in your > >> > > > case this should not matter, it does matter for other users of > >> > > > protobuf. The hashcode function would be better served by being > >> > > > implemented correctly from state data for the descriptor. > >> > > > Additionally, in generated code it seems that this hashcode could be > >> > > > pre-computed by the compiler and Descriptor.hashcode() could return a > >> > > > constant integer - which would be much more efficient than any other > >> > > > method. > > >> > > > On May 11, 3:02 pm, Jay Booth <[email protected]> wrote: > > >> > > > > It can be legitimate, especially in the case of Object.hashCode(), > >> > but > >> > > > > it's supposed to be in sync with equals() by contract. As it > >> > > > > stands, > >> > > > > two objects which are equal() will produce different hashes, or the > >> > > > > same logical object will produce different hashes across JVMs. > >> > > > > That > >> > > > > breaks the contract.. if the equals() method simply did return > >> > (other > >> > > > > == this), then it'd be fine, albeit a little useless. > > >> > > > > I created an issue and posted a 1-liner patch that would eliminate > >> > the > >> > > > > problem by using getClass().getName().hashCode() to incorporate > >> > > > > type > >> > > > > information into the hashCode without depending on a Descriptor > >> > > > > object's memory address. > > >> > > > > On May 11, 12:01 am, Dmitriy Ryaboy <[email protected]> wrote: > > >> > > > > > Hi Jay, > > >> > > > > > I encountered that before. Unfortunately this is a legitimate > >> > > > > > thing > >> > to > >> > > > > > do, as documented in Object.hashCode() > > >> > > > > > I have a write-up of the problem and how we wound up solving it > >> > (not > >> > > > > > elegant.. suggestions welcome) here: > >> >http://squarecog.wordpress.com/2011/02/20/hadoop-requires-stable-hash... > > >> > > > > > D > > >> > > > > > On Mon, May 9, 2011 at 8:25 AM, Jay Booth <[email protected]> > >> > wrote: > >> > > > > > > I'm testing an on-disk hashtable with Protobufs and noticed > >> > > > > > > that > >> > with > >> > > > > > > the java generated hashcode function, it seems to return a > >> > different > >> > > > > > > hashcode across JVM invocations for the same logically > >> > > > > > > equivalent > >> > > > > > > object (tested with a single string protobuf, same string for > >> > both > >> > > > > > > instances). > > >> > > > > > > Is this known behavior? Bit busy right now backporting this to > >> > work > >> > > > > > > with String keys instead but I could provide a bit of command > >> > line > >> > > > > > > code that demonstrates the issue when I get a chance. > > >> > > > > > > Glancing at the generated hashcode() function, it looks like > >> > > > > > > the > >> > > > > > > difference comes from etiher getDescriptorForType().hashCode() > >> > > > > > > or > >> > > > > > > getUnknownFields().hashCode(), both of which are incorporated. > > >> > > > > > > -- > >> > > > > > > You received this message because you are subscribed to the > >> > Google Groups "Protocol Buffers" group. > >> > > > > > > To post to this group, send email to [email protected]. > >> > > > > > > To unsubscribe from this group, send email to > >> > [email protected]. > >> > > > > > > For more options, visit this group athttp:// > >> > groups.google.com/group/protobuf?hl=en. > > >> > -- > >> > You received this message because you are subscribed to the Google Groups > >> > "Protocol Buffers" group. > >> > To post to this group, send email to [email protected]. > >> > To unsubscribe from this group, send email to > >> > [email protected]. > >> > For more options, visit this group at > >> >http://groups.google.com/group/protobuf?hl=en. > > >-- > >You received this message because you are subscribed to the Google Groups > >"Protocol Buffers" group. > >To post to this group, send email to [email protected]. > >To unsubscribe from this group, send email to > >[email protected]. > >For more options, visit this group at > >http://groups.google.com/group/protobuf?hl=en. > > -- > You received this message because you are subscribed to the Google Groups > "Protocol Buffers" group. > To post to this group, send email to ... > > read more » -- You received this message because you are subscribed to the Google Groups "Protocol Buffers" group. 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